The invention relates to a pressure-medium cylinder with a measuring system for determining the position of the piston rod relative to a reference point, and having further features.
A pressure-medium cylinder such as this is known from EP 0 618 373 B1. The measuring system for position determination is in this case designed such that only a relative position, that is to say only a movement distance, can be detected, without it being possible to identify the absolute position of the piston rod. The piston rod in the known pressure-medium cylinder has a regular code track of successive code elements of different binary states, which are formed by circumferential grooves of specific width and by circumferential plateaus, whose widths are the same as those of the grooves, between two grooves. In order to protect the piston rod, which is composed of steel in the base material, and to produce a smooth surface, a thin ceramic layer is applied to the base material and is finally processed by grinding and polishing.
The measuring system is a system based on a galvanomagnetic effect and has, in a fixed position with respect to the housing of the pressure-medium cylinder, a permanent magnet which produces a magnetic field and two magnetoresistive sensor elements, which are located between the permanent magnet and the piston rod and which are subjected to alternating magnetic field strengths during the movement of the piston rod as a result of the grooves and plateaus as they move past the sensor elements, and which accordingly emit a sequence of alternating output signals. Two sensor elements are provided, which are at a distance from one another in the longitudinal direction of the piston rod that is equal to an integer multiple of the pitch of the code track plus or minus a quarter of the pitch, so that high resolution is possible.